Technical Field
Embodiments describe herein relate to a method for processing a radioactive liquid waste containing boron.
Description of the Related Art
The common method for processing a liquid waste containing boron compound as the main component produced in a nuclear power plant such as pressurized water reactor is carried out by neutralizing the liquid waste using sodium hydroxide and subsequently solidifying mainly with a cement or asphalt.
However, the asphalt involves a fire hazard when heated and poor chemical adsorption of the radionuclide. For this reason, the cement solidification is the mainstream method in new plants.
In the cement solidification, however, the presence of boron and boron compounds retards a setting and hardening reaction of cement and consequently causes a notable hindrance in hardening and deteriorated strength. To deal with these issues, various techniques have been studied such as the solidification with the addition of a calcium hydroxide, or the like, to be a pretreatment agent from the perspective of solidifying a boric acid liquid waste with a cement while enhancing the volume reduction of the waste.
Patent Document 1 (Japanese Patent Laid-Open No. 10-90490), for example, discloses a method wherein a boric acid-containing liquid waste (sodium borate solution) is heated and concentrated at 90° C. or more, cooled to 60° C. or less to allow sodium borate to precipitate, subsequently a blast-furnace cement is added and kneaded, and the obtained kneaded mixture is discharged to a 200 L drum.
Patent Document 2 (Japanese Patent Laid-Open No. 11-72593) discloses a method wherein boric acid or a borate solution is adjusted to pH 7 to 10, mixed with a powder such as a divalent or higher valent metal oxide, hydroxide, salt, cement, or slug to form a slurry, and the slurry is adjusted to have a water content of 40% or less and hardened.
Patent Document 3 (Japanese Patent Laid-Open No. 2-208600) proposes a method wherein a calcium hydroxide is added to a boric acid liquid waste to form a dry powder, which is subsequently solidified by compaction or with a resin.
Patent Document 4 (Japanese Patent Laid-Open No. 2010-2378) proposes a method comprising boric acid insolubilization, which includes a first step of controlling a boric acid-containing liquid waste by adding an alkali metal element compound and a second step wherein a temperature is subsequently increased to a predetermined temperature of 85° C. or higher and an alkali earth metal compound is added thereto and stirred, and drying the treated liquid waste to obtain a powdered waste, which is then cement solidified.
Patent Document 5 (Japanese Patent Laid-Open No. 2001-97757) proposes a method for cement-solidifying a dry-powdered boric acid liquid waste, wherein a boric acid liquid waste is dried without pretreatment such as insolubilization, and the produced dry-powder is solidified with cement by adding a sodium aluminate to be a cement hardening promoter and lithium hydroxide to be an aid.
When a solidifying material such as a cement or slug is added to and mixed with a solution of boric acid liquid waste concentrated to supersaturation as in the conventional techniques described above, sodium borate absorbs water and forms a hydrate whereby the plasticity is lost very quickly to cause false setting. Consequently, sufficient hydration reaction between a solidifying material such as a cement and water is prevented and a strength of the obtained solidified product is deteriorated.